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A soldier opened fire on fellow service members at the Fort Hood Army post on Wednesday afternoon, killing three and injuring 16 before turning the gun on himself, military officials say. (Published Wednesday, Apr 2, 2014)

NBC News identified the deceased gunman as 34-year-old enlisted Army soldier Ivan A. Lopez, but military officials declined to name him Wednesday night, pending notification of family members. Military officials said the gunman's rampage was stopped after he was confronted by a female military police officer, which led to the shooter turning the gun on himself.

Officials said there was no indication the shooting was terrorism-related. The gunman's motive remained unknown, officials said, although NBC News reported that the rampage may have resulted from an argument with other soldiers in the motor pool.

More than four hours after the shooting, all-clear sirens sounded as the lockdown at the post was lifted. Hundreds of cars began streaming from the giant complex, many including children who had been kept locked-down in schools since gunshots were first reported at about 4:30 p.m.

President Obama on Fort Hood Shooting

President Barack Obama responds to Fort Hood Shooting; said "we are going to get to the bottom of what happened." (Published Wednesday, Apr 2, 2014)

The names of the victims have not yet been released, though officials confirmed that all are military personnel. Their names will be released 24 hours after all family have been notified.

Portrait of Gunman Begins to Emerge

Fort Hood Tragedy: Deadly Shooting's Aftermath

The gunman had served four months in Iraq in 2011 and had known mental health issues, Fort Hood's commanding general Lt. Gen. Mark Milley told reporters Wednesday evening.

He was under diagnosis for post-traumatic stress disorder but had not been officially diagnosed with PTSD, Lt. Gen. Milley said. He was undergoing behavioral health care for depression and anxiety, had a self-reported traumatic brain injury and was not physically injured in combat.

The gunman was armed with a single weapon, a .45-caliber Smith & Wesson handgun he had recently purchased, Milley said.

NBC News also reported that the gunman served with the Puerto Rican Army National Guard and was an E4 in the U.S. Army.

NBC 5 spoke with Lopez's neighbors in Killeen Wednesday night. They said Lopez moved into the apartment complex about three weeks ago with his wife, and young daughter.

"They seemed real sweet," said neighbor Xanderia Morris.

Morris said when Lopez's name was announced on TV news reports, his wife came out of her apartment hysterical. Morris said she comforted her until authorities arrived a short time later and escorted her away. Their daughter apparently left with relatives, Morris said.

Temple Hospital Takes Fort Hood Patients

Baylor Scott & White Hospital in Temple confirmed it had a command center in place and received nine patients from the post.

The other six patients still listed at the hospital are in serious condition as of 7 a.m. Thursday, according to Papaconstantinou. Doctors are hopeful the patients will be upgraded to fair condition later today.

“What she did was heroic,” Milley told reporters late Wednesday. “She did her job and she did exactly what we’d expect from U.S. Army military police.”

Investigation Continues

Lopez had been assigned to the 13th Sustainment Command (Expeditionary). The suspected gunman had fired shots at individuals in the 1st Medical Brigade, Milley said Wednesday evening.

Milley said the shooter then left that building, got into a vehicle and continued firing. He then went to another building at the post, went inside and opened fire. When a military police officer confronted him, the gunman put his gun to his head and pulled the trigger, Milley said.

Nothing is being ruled out as a possible cause of the attack, Milley said. The investigation is being conducted with the support of multiple federal agencies, the Texas Rangers, The Texas Department of Public Safety, military police, Army CID, the Killeen Police Department and the Harker Heights Police Department.

During the lockdown of the base, officials with the Bell County Sheriff's Office and Texas Department of Public Safety were called in to help to secure the perimeter of the largest active duty armored post in the U.S. Armed Services.

Obama, Gov. Perry Respond

President Barack Obama addressed the shooting in brief remarks in Chicago, where he was attending a fundraiser Wednesday night.

"We're following it closely. The situation is fluid right now ... I want to just assure all of us we are going to get to the bottom of exactly what happened," he said. "We're heartbroken something like this might have happened again."

Charlotte Spencer was picking up her 4-year-old son from soccer practice on the Fort Hood post when Wednesday’s shooting occurred.

Spencer said her son had just climbed into the car when a woman came over a loudspeaker telling everyone to shelter in place immediately.

“The siren came over, and she was like, ‘This is an emergency. Get in your homes, lock your doors, lock your windows, turn off your AC units and turn off your heaters if you have them running. Just stay in place. This is an active emergency,’” Spencer described.

Spencer said she tried to delicately explain the all-too-familiar situation to her young son.

Antonio Ortiz, 30, who lives a quarter of a mile from the east gate of Fort Hood, told NBC News he heard a commotion and went outside to hear alarms going off and announcements for people to stay inside. He went back in and turned on the TV news, then soon after heard a barrage of gunshots.

"It sounded powerful,” Ortiz said, adding that while it seemed to be coming from the base, he couldn't rule out the possibility someone in the civilian neighborhood was shooting.

Tayra DeHart, 33, told The Associated Press she had last heard from her husband, a soldier at the post, that he was safe, but that was hours earlier.

"The last two hours have been the most nerve-wracking I've ever felt. I know God is here protecting me and all the soldiers, but I have my phone in my hand just hoping it will ring and it will be my husband," DeHart said.

Tragic History at Fort Hood

In November 2009, 13 people were killed and more than 30 others injured when Maj. Nidal Hasan, an Army psychiatrist, opened fire on dozens of people at the post. Hasan was paralyzed during an exchange of gunfire and, in late 2013, was sentenced to death. He is awaiting execution.

Hasan's rampage isn't the most recent mass shooting at a U.S. military installation. Last September, a lone gunman with ties to North Texas, Aaron Alexis, killed 12 when he opened fire at the Washington Navy Yard.

Fort Hood covers a total of 340 square miles and supports multiple units, a corps headquarters and a robust mobilization mission. It is home to two full divisions, the 1st Cavalry Division and 4th Infantry Division (Mechanized) and 12 additional units.

Around 50,000 soldiers work at Fort Hood, and there are an additional 150,000 civilians who support the base.

The post is about 60 miles north of the capital city of Austin, 50 miles south of Waco, 160 miles south of Dallas and 150 miles north of San Antonio.